It is known to concentrate liquid in tube-bundle evaporators by passing the material to be concentrated or subjected to evaporation through the heating and evaporator tubes of the tubes of the tube bundle while the tube is heated from the exterior and the material to be evaporated releases steam or vapor which is recovered from the tube bundle. An evaporator of this type comprises a heating and evaporator tube bundle which can be provided, as described in German open application (Offenlegungsschrift) DT-OS 19 33 462, with internal troughs or corresponding channels designed for a constant tube perimeter, to afford a progressive increase in the tube cross section in the flow direction.
It is also known to provide heating and evaporator tubes in falling-stream and climbing film evaporators which have helical troughs extending around the tube, the troughs or channels terminating before the ends of these tubes and each tube having three or four such troughs (see German patent DT-PS 15 19 658).
These also permit aqueous liquids to be evaporated to relatively high final concentrations although the evaporation processes are not fully satisfactory. For example, since the material to be evaporated achieve an extremely high viscosity during the process, interruption of the latter is a problem which has frequently arisen. Furthermore, the heating surfaces tend to become soiled and to accumulate deposits so that operation of the unit must be interrupted to permit cleaning and removal of these deposits.